Evidence for cross formational hot brine flow from integrated 87Sr/86Sr, REE and fluid inclusions of the Ordovician veins in Central Tarim, China

Rare earth elements, delta S-34, Sr-87/Sr-86 and delta O-18 were determined, and fluid inclusions were analyzed of fracture filling barite, anhydrite, fluorite, calcite and quartz from Upper Ordovician carbonates in Central Tarim, China. The aim was to assess the origin, evolution and flow of fluid in the fractures. There was mixing of relatively hot Ba-rich and Sr-87-rich fluid with in situ basinal water and fresh water which was relatively cool and more radiogenic. The hotter fluid is hydrothermal as indicated by homogenization temperatures (HTs) about 20 degrees C higher than the formation, and was most likely derived from Precambrian to Lower Cambrian clastic rocks or/and from the basement through faults. This is because almost all of Sr-87/Sr-86 ratios of late stage fracture-fillings (mainly 0.70889-0.71036) and present brines are higher than that of Upper Ordovician limestones. The hydrothermal fluid was enriched in F-, Eu2+, Ba2+, Sr2+ and Zn2+ during migration as indicated by the positive relationships of Eu to Sr, Ba and Zn. When the hydrothermal fluid mixed with local basinal water, it precipitated isotopically-normal S anhydrite, fluorite and calcite with positive Eu anomalies. The precipitation of these Ca-minerals was followed by thermochemical SO4 reduction (TSR) of dissolved sulphates by hydrocarbons. It was likely that only part of the SO42- was reduced and the generated H2S leaked Out, thus barite deposited from 4 residual SO42 has abnormally high delta S-34 values from 42 parts per thousand to 47 parts per thousand and the generated H2S 4 is isotopically light from 15 parts per thousand to 18 parts per thousand. Subsequently, hydrothermal fluid was likely to have mixed with relatively cool and more Sr-87-rich freshwater, resulting in precipitation of quartz and fluorite with salinities as low as 0.5 wt% NaCl equiv. and Sr-87/Sr-86 ratios tip to 0.71036. (c) 2008 Elsevier Ltd. All rights reserved.